To ignore insertions and deletions of lines that match a
grep-style regular expression, use the
--ignore-matching-lines=regexp (-I regexp) option.
You should escape
regular expressions that contain shell metacharacters to prevent the
shell from expanding them. For example, ‘diff -I '^[[:digit:]]'’ ignores
all changes to lines beginning with a digit.

However, -I only ignores the insertion or deletion of lines that
contain the regular expression if every changed line in the hunk—every
insertion and every deletion—matches the regular expression. In other
words, for each nonignorable change, diff prints the complete set
of changes in its vicinity, including the ignorable ones.

You can specify more than one regular expression for lines to ignore by
using more than one -I option. diff tries to match each
line against each regular expression.